This invention relates to lithography, and more particularly, to masks used in lithography and mask support structures and their preparation.
As the size of electronic components used in integrated circuits has shrunk and their density in circuits increased, there has been growing interest in electron beam lithography. In electron beam lithography, an electron beam is used to write patterns on an electron beam resist layer formed over a top surface of the semiconductor wafer that is the workpiece for the electron beam lithography. A particular form of electron beam lithography of special interest for the invention is described as Scattering With Angular Limiting Projection Electron Beam Lithography (xe2x80x9cScalpelxe2x80x9d).
Scalpel Is an electron beam lithography technique that typically has employed masks in which the mask used for control in the irradiation of the workpiece has been formed as a thin patterned coating supported on a thin membrane. Scalpel has been described in various publications.
The requirements for such a membrane-mask combination are quite demanding. Typically, it needs to be planar, relatively easy to make, and comparatively rugged. A particular problem has been the forming of the membrane that is to support the coating in which the mask pattern is formed. The membrane needs to be of relatively large area, typically at least several inches on a side, to be practical for use in large scale manufacture. It needs also to be very thin, typically no thicker than about 1000 Angstroms in thickness, to permit electrons that pass through the mask to penetrate it easily without excessive scattering or experiencing excessive loss in electron beam energy. Moreover, it also needs to remain planar with inappreciable sag in use so that it continues to support the mask uniformly over its entire area.
To insure that the membrane remains sag-free, it is generally the practice to suspend the membrane by a substrate that provides a suitable underlying support grillage, typically consisting of major struts and minor struts (ribs)
Hitherto, for forming the grillage for supporting the membrane that supports the mask coating, a layer of silicon nitrite was deposited over the top surface of a silicon wafer and then the silicon wafer was etched to leave a portion thereof to provide the supporting grillage for the nitride coating that was to serve as the membrane. However, this has proven difficult.
One problem with this approach has been the difficulty of maintaining, during the shaping of the silicon wafer, the mechanical stability of the grillage left to support nitride the membrane. The physical dimensions of commercially available silicon wafers limits formation of thin struts needed.
The present invention involves a better way to prepare a mask useful in electron beam lithography.
In a first aspect the invention is a method for forming a mask assembly for use in lithography. The method comprises the steps of: forming a support structure that comprises a substrate that includes a plurality of windows filled with a temporary fill; forming over the filled-windowed substrate a mask; and removing the temporary fill.
In a second aspect the present invention is a method for forming a mask assembly for use in lithography. The method comprises the steps of: forming a support structure that comprises a substrate that includes a plurality of windows filled with a temporary fill; forming over the filled-windowed substrate a membrane layer for supporting the mask layer; forming a mask layer over the membrane layer; and removing the temporary fill.
In a third aspect the invention is a method of forming a mask assembly that comprises the steps of: forming in a substrate a supports structure, which includes major and minor struts that define an array of windows in a two-dimensional array of rows and columns, by successive rounds of cutting in the substrate a fraction of the total window area to be formed; filling such fraction of windows with temporary fill before the succeeding round of cutting and filling until all the window areas are cut and filled; forming a membrane layer over a top surface of the support structure; forming a mask layer over the membrane layer; and removing the fill from the windows.
In a fourth aspect the present invention is a method of forming a mask assembly for use in electron beam lithography that comprises the steps of: forming in a substrate a first set of spaced-apart windows; filling the windows with a temporary fill; forming in the substrate a second set of windows in the spaces between the first set of windows for forming with the first set a two-dimensional array of windows arranged in row and columns; filling the second set of windows with a temporary fill; depositing over the filled-windowed substrate a layer suitable for supporting a mask; depositing over the last-mentioned layer a layer suitable for providing a mask; patterning the last-mentioned layer to form a mask, and removing the temporary fill from the windows, whereby the mask layer is free of underlying substrate.
In a fifth aspect the present invention is a method for forming a mask assembly comprising the steps of: forming by use of a mold a support structure that defines an array of windows arranged in rows and columns; filling the openings with a temporary fill; forming over the support structure a membrane layer; forming over the membrane layer a patterned mask; and removing the temporary fill.
In a sixth aspect the invention is a method of forming a mask support structure that comprises the steps of: forming in a substrate a first set of spaced apart windows; filling the first set of windows with a temporary fill; forming in the substrate a second set of windows located in portions of the substrate between the first set of filled windows; and filling the second set of windows with a temporary fill.
In a seventh aspect the invention is a method of forming a mask support structure that comprises the steps of: placing in a mold which is shaped to facilitate the formation of a support structure a plurality of parallel minor struts; and forming in the mold a mask support structure that comprises a frame and plurality of major struts that are orthogonal and attached to the minor struts with the major and minor struts defining a plurality of windows arranged in a two dimensional array of rows and columns.
From a product aspect, the invention is the product of the various methods described.
The invention will be better understood from the following more detailed description in conjunction with the accompanying drawing and claims.